1. Field of the Invention
The present invention relates to a method for manufacturing a three-dimensional photonic crystal, which can have a three-dimensional refractive-index periodic structure, and devices using the same.
2. Description of the Related Art
Yablonovitch et al. have introduced the idea of controlling the transmission and reflection of electromagnetic waves using fine structures having dimensions smaller than the wavelengths thereof (Physical Review Letters, Vol. 58, pp. 2059 (1987)). That is, the transmission and reflection of an electromagnetic wave can be controlled with an array of the fine structures. If the wavelength of the electromagnetic wave is as short as the wavelength of light, the transmission and reflection of light can be controlled with the fine structures. Materials having the fine structures are known as photonic crystals. A reflecting mirror that has a reflectivity of 100%, that is, no optical loss over a certain range of wavelengths can be probably prepared using a photonic crystal. Such a range of wavelengths is referred to as “photonic band gap” analogous to the energy gap of known semiconductors. A three-dimensional periodic fine structure has a photonic band gap in which light cannot propagate in any direction. Such a photonic band gap is hereinafter referred to as “complete photonic band gap”. Structures with a complete photonic band gap can be used for various applications, for example, the control of the spontaneous emission of light-emitting devices. This leads to the possibility of novel functional devices.
There is an increasing demand for functional devices having structures with complete photonic band gaps over a wide wavelength range.
U.S. Pat. Nos. 6,392,787, 6,597,851, and 5,335,240 disclose structures with a photonic band gap.
In general, it can be difficult to prepare three-dimensional periodic structures with a small size. Hence, such three-dimensional periodic structures rarely operate at optical wavelengths less than several micrometers in a vacuum.
A layer-by-layer structure (hereinafter referred to as an LBL structure) can be prepared by stacking layers including refractive-index periodic structures. Some LBL structures have been prepared and evaluated. The evaluation has shown that the LBL structures have photonic band gaps. Typical examples of such an LBL structure include a structure discussed in U.S. Pat. No. 6,597,851 and a woodpile structure, shown in FIG. 9, discussed in U.S. Pat. No. 5,335,240.
FIG. 9 is an illustration of the woodpile structure. The woodpile structure represented by reference numeral 900 includes four layers: a first layer 901, a second layer 902, a third layer 903, and a fourth layer 904. The layers are arranged in the Z direction in FIG. 9 and form periodic units. The first to fourth layers 901 to 904 each include a plurality of rectangular rods 910, of which the width and height are represented by W and H, respectively, and which are arranged at an equal pitch represented by P. The rectangular rods 910 included in the first layer 901 extend in the Y direction and the rectangular rods 910 included in the second layer 902 extend in the X direction. The rectangular rods 910 included in the third layer 903 extend in the Y direction and are displaced from those of first layer 901 by P/2 in the X direction. The rectangular rods 910 included in the fourth layer 904 extend in the X direction and are displaced from those of the second layer 902 by P/2 in the Y direction. The woodpile structure has two periodic units. In the woodpile structure, all of the rectangular rods 910 contain a first medium and portions other than the rectangular rods 910 contain a second medium having a refractive index different from that of the rectangular rods 910. The pitch between the rectangular rods 910, the width and height of the rectangular rods 910, the refractive index of the first medium, and the refractive index of the second medium are determined such that the woodpile structure serves as a photonic crystal exhibiting a photonic band gap in a desired wavelength range. If the first medium has a refractive index of 3.309, the second medium has a refractive index of 1, and the rectangular rods 910 have a width of 0.30 P and a height of 0.30 P, the following result can be obtained by the calculation of the photonic band structure by a plane-wave expansion method: the woodpile structure has a complete photonic band gap at a normalized frequency (an angular frequency normalized with P) of 0.362 to 0.432. That is, if the pitch between the rectangular rods 910 is 600 nm, the woodpile structure has a complete photonic band gap at a wavelength of 1,389 to 1,657 nm.
Various methods for manufacturing woodpile structures are discussed in, for example, U.S. Pat. Nos. 5,406,573 and 5,998,298.
U.S. Pat. No. 5,406,573 discusses a method for manufacturing a woodpile structure by a wafer fusion technique. U.S. Pat. No. 5,998,298 discusses a method for manufacturing a woodpile structure by a procedure in which the formation, deposition, and polish of each refractive index periodic structure are repeated.
The method discussed in U.S. Pat. No. 5,406,573 will now be described with reference to FIGS. 10A to 10C. As illustrated in FIG. 10A, a transfer layer 1002, an etching stop layer 1003, and a transfer substrate 1004 are deposited on a substrate 1001, having a periodic refractive index pattern formed by an etching process, in that order and then fused to each other. As illustrated in FIG. 10B, the etching stop layer 1003 and the transfer substrate 1004 are etched off and the periodic refractive index pattern is transferred to the transfer layer 1002. The fusion of layers and substrates, the removal of substrates, and the formation of patterns are repeated, whereby a multilayer structure shown in FIG. 10C is prepared.
The method discussed in U.S. Pat. No. 5,998,298 will now be described with reference to FIGS. 11A to 11D. As illustrated in FIG. 11A, a thin-film layer 1102 is formed on a substrate 1101 by the vapor deposition of a first medium. As illustrated in FIG. 11B, the thin-film layer 1102 is etched so as to have a periodic refractive index pattern and a second medium 1103 is then deposited on the thin-film layer 1102 such that hollows in the periodic refractive index pattern can be filled with the second medium 1103. As illustrated in FIG. 11C, the second medium 1103 is polished. The formation of thin-film layers, the formation of periodic refractive index patterns, the deposition of media, and polishing are repeated, whereby a multilayer structure shown in FIG. 11D is prepared.
A periodic variation of the dielectric constant of a photonic crystal results in a photonic band gap. Therefore, in order to manufacture a photonic crystal with a photonic band gap at desired wavelengths, the three-dimensional periodicity of the photonic crystal can be controlled. In a method for manufacturing a three-dimensional photonic crystal by stacking layers each, which can have a periodic refractive index structure, the following items need to be controlled: the periodicity of the periodic refractive index structure, configurations of members included in the periodic refractive index structure, and the thickness of the layers. In a photonic crystal, which can have the woodpile structure shown in FIG. 9, if the actual width of rectangular rods included in this photonic crystal is 200 nm although the design width thereof is 180 nm, a wavelength range corresponding to a photonic band gap of this photonic crystal is shifted by about 60 nm from a design wavelength range. Alternatively, if the actual height of these rectangular rods is 200 nm although the design height thereof is 180 nm, this wavelength range is shifted by about 160 nm from the design wavelength range.
A periodic structure manufactured by a known method has a dimensional deviation that is equal to the sum of the thickness deviations of layers included in the periodic structure. Therefore, in order to control the deviation of the actual center wavelength of a photonic band gap (hereinafter referred to as a photonic band gap center wavelength) from the design center wavelength within, for example, 20 nm, the thickness deviation of each layer should be controlled within 2.5 nm.
In order to reduce the deviation of the actual photonic band gap center wavelength from the design photonic band gap center wavelength, an allowance of the thickness deviation of each layer should be controlled to be very small; hence, it can be very difficult to manufacture a three-dimensional photonic crystal operating at desired wavelengths. In the method for manufacturing the multilayer structure shown in FIG. 10, the sum of the following deviations should be controlled within a predetermined allowance: the thickness deviation of each transfer layer 1002, the thickness deviation of each substrate 1001, and the etching deviation of the transfer layer 1002. Hence, it can be difficult to manufacture this multilayer structure. In the method for manufacturing the multilayer structure shown in FIG. 11, the sum of the following deviations should be controlled within a predetermined allowance: the thickness deviation and polishing deviation of each thin-film layer 1102 and the thickness deviation of each substrate 1101. This method is low in yield and it can be difficult to manufacture this multilayer structure.